1. Field of the Invention
The present invention relates generally to switching circuits, and more particularly, to photocell switching circuits suitable for use in hazardous environments.
2. State of the Art
It is often necessary to install electrical equipment in hazardous areas such as areas which contain explosive gases or dusts. In hazardous areas, such as explosive environments, any source of energy can touch off an explosion when flammable gases or combustible dusts are mixed in proper proportion with air. Equipment such as electrical switching circuits (e.g., relays) can produce arcs or sparks in normal operation (i.e., when contacts are opened and closed) that can easily cause ignition of flammable gases or combustible dusts. Further, any device which produces heat can produce surface temperatures which may exceed safe limits of a flammable atmosphere and result in ignition.
Because electrical safety is of critical importance, electrical installations must be designed to prevent accidental ignition of flammable liquids, vapors or dusts released to the atmosphere. The National Fire Protection Association, Inc. (NFPA) 70-1993, National Electrical Code, Copyright 1992, NFPA, Quincy Massachusetts 02269 sets forth definitions to clarify what constitutes hazardous areas and constraints imposed on such areas to ensure safe operation of electrical equipment therein.
Article 500 entitled "Hazardous (Classified) Locations" of the 1993 National Electric Code covers requirements for electrical equipment in locations where fire or explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dust, or ignitable fibers or flyings. Class I atmospheric hazards are divided into two divisions. Division 1 covers locations where flammable gases or vapors may exist under normal operating conditions, under frequent repair or maintenance operations, or where breakdown or faulty operation of process equipment might also cause simultaneous failure of electrical equipment. Division 2 covers locations where flammable gases, vapors or volatile liquids are handled either in a closed system, or confined within suitable enclosures, or where hazardous concentrations are normally prevented by positive mechanical ventilation. Areas adjacent to Division 1 locations are also considered Division 2 areas.
In Class I, Division 1 and 2 locations, conventional electrical equipment such as switching circuits (e.g., relays, contacters and switches) having arcing contacts must be enclosed in explosion-proof housings or enclosures. The enclosures must prevent ignition of an explosive gas or vapor which may surround the enclosure such that sparks inside the enclosure are prevented from igniting a larger explosion outside the enclosure. Such an explosion-proof enclosure must be formed of adequate strength to withstand, for example, a hydrostatic pressure test of four times the maximum pressure from an explosion within the enclosure. Further, such an enclosure-must be fabricated as a "flame-tight" housing, thus requiring that joints or flanges be maintained within narrow tolerances.
The requirement for enclosures suitable for use in Class I, Division 1 and 2 locations can impose significant design constraints and production costs on electrical equipment to be used in these locations. For example, electrical circuits are used to control lighting fixtures which are to be used in Class I, Division 1 and 2 locations. In locations where explosive gases or vapors exist, bare lamps or non-explosion proof enclosed equipment constitute hazards since bare lamps or exposed electrical equipment can cause an explosion. A typical switching circuit used to control the ON/OFF operation of such lighting fixtures includes a photocell which responds to ambient light so that the lamp is turned ON only when insufficient ambient light exists in the hazardous location. Because of their insensitivity to noise, photocells have been used in applications such as Class I, Division 2 environments wherein explosion-proof enclosures are typically required.
Conventional photocell switching circuits used in hazardous locations include general use photocells in conjunction with an electro-mechanical relay to provide a switching function. However, the potential for arcing and sparks generated by an electro-mechanical relay requires that the electro-mechanical relay be housed in an approved enclosure separate from or integrated with the photocell. The need for an electro-mechanical relay housed in an explosion-proof enclosure results in added switching circuit complexity. Further, the explosion-proof enclosure needed to house the electro-mechanical relay requires external seals, and is costly to manufacture.
U.S. Pat. No. 4,658,129 (Fan) discloses a general purpose photoelectric control device which includes a trigger unit 2. The trigger unit 2 includes a silicon controlled rectifier (SCR) 21 having a gate G2 connected to a photoelectric sensing element 13 of a sensing unit 1 via a capacitor 22. A first switching unit 3 supplies a rectified voltage to the sensing unit 1. A second switching unit 4 includes a TRIAC 41 which turns a load unit 5 on and off in response to light intensity changes detected by the sensing unit 1.
The general purpose photoelectric control device of U.S. Pat. No. 4,658,129 is unsuitable for use in hazardous locations. For example, the switching devices of this patent (e.g., SCR 21 and TRIAC 41) are temperature dependent devices (i.e., their switching threshold fluctuates with changes in temperature). The control device of this patent is analog in nature (i.e., the SCR 21 is gated with an analog signal) and is only calibrated, if at all, for use with a specific temperature (i.e., no temperature compensation is provided). Further, the disclosed control device is only useable with a limited input voltage range (i.e., no voltage regulation is provided).
Accordingly, it would be desirable to eliminate any need for using remote photocells to control relays housed in separate explosion-proof housings for Class I, Division 1 and 2 environments by providing a less complex, more cost-effective switching circuit suitable for high temperature operation in hazardous environments. For example, such a switching circuit should be useable in high intensity discharge (HID) lighting wherein electrical components of the switching circuit must be suitable for operation at 90.degree. C. ambient without degradation of performance. Further, it would be desirable to provide a switching circuit which operates independently of temperature variations and which is useable with a wide range of voltage inputs.